1. Field of the Invention
The invention relates generally to the field of drill bits used to drill wellbores through earth formations. More specifically, the invention relates to hardfacing structures on such bits, and methods for applying hardfacing structures on these bits to reduce erosion of certain parts of a drill bit during wellbore drilling.
2. Description of the Related Art
Drill bits used to drill wellbores through earth formations generally are within one of two broad categories of bit types. One of the types of drill bits is known as xe2x80x9cfixed cutterxe2x80x9d bits, which generally include a bit body formed from steel or other high strength material, and a plurality of cutting elements disposed at selected positions about the bit body. The cutting elements may be formed from any one or combination of hard or superhard materials, including natural or synthetic diamond, boron nitride and tungsten carbide.
The other type of drill bits is known as xe2x80x9croller conexe2x80x9d bits, which include a bit body having one or more roller cones rotatably mounted to the bit body. The bit body is typically formed from steel or other high strength material. The roller cones are also typically formed from steel or other high strength material, and have a plurality of cutting elements disposed at selected positions about the cones. The cutting elements may be formed from the same base material as is the cone, as in so-called xe2x80x9cmilled toothxe2x80x9d bits. Other roller cone bits include cutting elements called xe2x80x9cinsertsxe2x80x9d which are press (interference) fit into sockets formed into the cone material. The inserts may be made from tungsten carbide, natural or synthetic diamond, boron nitride, or any one or combination of hard or superhard materials.
It is known in the art to apply hardfacing to the base material from which the cones are formed in roller cone bits. The hardfacing can be applied in the form of special erosion protection inserts used in addition to the cutting elements. See, for example, U.S. Pat. No. 3,952,815 issued to Dysart. Another way known in the art to use hardfacing to protect a roller cone is described in U.S. Pat. No. 5,291,807 issued to Vanderford. The method in the Vanderford ""807 patent includes marking the face of a roller cone by masking or etching, applying hardfacing material, such as tungsten carbide, in the form of a powder, and heating the cone to bond the hardfacing powder to the cone. U.S. Pat. Nos. 3,461,983 and 3,513,728 issued to Hudson includes drilling holes (the sockets) in the cone prior to hardface application, plugging the holes, and then applying the hardface using a heating process. After heating to apply the hardfacing material, the plugs are removed and the inserts are pressed into the previously drilled sockets.
U.S. Pat. No. 5,348,770 issued to Sievers discloses a method for applying hardfacing to a cone which uses high velocity oxygen fuel (HVOF) spraying after the cone is formed. Forming the cone includes drilling the sockets for the inserts.
U.S. Pat. No. 4,396,077 issued to Radtke discloses a method for applying hardfacing to a fixed cutter bit, which includes generating an electric arc and spraying the arc-heated hardfacing material on to a substantially completely assembled bit structure.
Generally, the prior art methods for applying hardfacing to drill bits, and to roller cones in particular, require forming the bit or cone structure to which the hardfacing is to be applied prior to applying the hardfacing. A limitation of the prior art methods, generally speaking, is that they heat the structure to which the hardfacing is applied, after making the structure to which cutting elements are attached. Such heating may reduce the structural integrity of the attachment of the cutting elements to the bit structure. What is needed is a method for creating a bit structure which provides for application of hardfacing prior to making an attachment of the cutting elements to the bit structure.
The invention is a method for forming a drill bit structure to which a cutting element is to be affixed. The method includes applying a layer of hardfacing material to a basic bit structure, removing the layer of hardfacing material in a location to which the cutting element is to be affixed, and forming an attachment structure at the location.
In one embodiment, the removing is performed by electrode discharge machining. In another embodiment, the removing is performed by high pressure water erosion.
In one embodiment, the basic bit structure is a roller cone, and the cutting element is an insert adapted to be interference fit within a socket in the roller cone.
In this specific embodiment, the removing of the hardfacing is limited to creating an aperture in the hardfacing such that subsequent processing can occur to produce the cutting element receiving socket.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.